Over at least the past thirty-five years or so there has been a continuing interest in the ability to sense what computing systems are doing from afar. A number of methods have been explored, including receiving inadvertent radio emissions, visible light examination, magnetic sensing, temperature sensing, multispectral analysis, and acoustic sensing. All the previous methods have been limited in a combination of precision, range, and general applicability. Known methods for observing target device characteristics and behavior during operation require some combination of software and/or hardware resident on the target device, knowledge of target device communication protocols and modalities, and/or close physical access to the target device. Current and past techniques thus require substantial a priori knowledge, are potentially invasive to target device operation, and are susceptible to erroneous or even malicious activities on the target device.